You can't really do true promiscuous mode in Bluetooth LE (BLE) without a software defined radio - and a lot more computing power - because BLE uses 40 RF channels for communication. Even Bluetooth sniffers will only listen to one channel at a time.
You can however listen to advertising packets from BLE devices in peripheral mode, count the individual MAC addresses seen and use that as a count of the number of people in the room (assuming everyone has a BLE device actively advertising).
You can do the above with the RM186, for example with some simple modification of the BLE code I posted:
Just count the unique periphAddr$ returned by BleScanGetAdvReport()
Be aware that for privacy reasons some BLE devices use a private address which keeps changing so you might overcount it. Check the Bluetooth Core specification for details.